The growth or repair of mature skeletal muscle relies upon a small, poorly characterized population of myogenic cells termed satellite cells due to their location peripheral to muscle fibers and beneath the surrounding basal lamina. Activated by growth factors, stress or injury, these normally quiescent cells proliferate, differentiate, and become a stable part of long-lived muscle fibers. Satellite cells are critical for muscle homeostasis and have a direct impact on muscle repair, disease progression, and aging. Only small subsets of satellite cells have characteristics of muscle stem cells. Stem cells give rise to both myoblasts, which differentiate after a limited number of divisions, as well as more self-renewing stem cells that maintain a high proliferation potential. Quiescent muscle stem cells are difficult to study due to their sparse number and the lack of defining biochemical or molecular features. The objective of the proposed research is to identify characteristic features of muscle stem cells, and use these to study the role of stem cells in muscle growth and repair, particularly with respect to degeneration associated with muscular dystrophy. Recent work suggests that proteins that inhibit apoptotic cell death, such as Bcl-2, are expressed in self-renewing stem cells found in various tissues such as skin epithelium and intestine. We have found that a small subset of myogenic cells grown in vitro expresses Bcl-2 in a pattern consistent with them acting as myogenic stem cells. In addition, we found that satellite cells from Bcl-2-deficient animals give rise to muscle colonies that are significantly smaller than their wild-type siblings. To extend these observations, muscle cells will be examined, both in vitro and in vivo, to determine whether Bcl-2 expression is a characteristic of muscle stem cells. Apoptosis-related gene expression (Bcl-2, and others) will also be studied in mdx mice, a model for Duchenne muscular dystrophy in which muscles undergo an acute phase of degeneration, extensive stem cell activation and repair. Transgenic mice will be made which express Bcl-2 driven by the muscle- specific MRF4 promoter. These mice will be mated with mdx mice to determine if overexpressed Bcl-2 can inhibit muscle degeneration or affect the size of stem cell populations present in diseased muscle. In summary, results should facilitate identification of muscle stem cells and determine the role of apoptosis regulators in muscle growth and homeostasis. Such information will significantly contribute to our understanding of muscle biology, and may lead to new therapeutic strategies for treatment of muscle disorders, muscle atrophy and better methods for muscle-based gene therapy.